U.S. patent number 10,319,597 [Application Number 15/880,506] was granted by the patent office on 2019-06-11 for semiconductor device with particular fin-shaped structures and fabrication method thereof.
This patent grant is currently assigned to UNITED MICROELECTRONICS CORP.. The grantee listed for this patent is UNITED MICROELECTRONICS CORP.. Invention is credited to En-Chiuan Liou, Yu-Cheng Tung.
United States Patent |
10,319,597 |
Liou , et al. |
June 11, 2019 |
Semiconductor device with particular fin-shaped structures and
fabrication method thereof
Abstract
A semiconductor device includes first fin-shaped structures and
second fin-shaped structures, which are separately disposed on a
semiconductor substrate. Each of the first and second fin-shaped
structures includes a base portion and a top portion protruding
from the top portion. The base portions of the second fin-shaped
structures are wider than the top portions of the second fin-shaped
structures, and the top portions of the second fin-shaped
structures are as wide as the top portions of the first fin-shaped
structures. Each second fin-shaped structure further includes a
recessed region on its sidewall.
Inventors: |
Liou; En-Chiuan (Tainan,
TW), Tung; Yu-Cheng (Kaohsiung, TW) |
Applicant: |
Name |
City |
State |
Country |
Type |
UNITED MICROELECTRONICS CORP. |
Hsin-Chu |
N/A |
TW |
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Assignee: |
UNITED MICROELECTRONICS CORP.
(Hsin-Chu, TW)
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Family
ID: |
57684054 |
Appl.
No.: |
15/880,506 |
Filed: |
January 25, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180151371 A1 |
May 31, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14820565 |
Aug 7, 2015 |
9922834 |
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Foreign Application Priority Data
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Jul 1, 2015 [TW] |
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104121374 A |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
21/76224 (20130101); H01L 21/823431 (20130101); H01L
21/845 (20130101); H01L 21/283 (20130101); H01L
21/308 (20130101); H01L 21/31 (20130101) |
Current International
Class: |
H01L
21/283 (20060101); H01L 21/8234 (20060101); H01L
21/84 (20060101); H01L 21/31 (20060101); H01L
21/308 (20060101); H01L 21/762 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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201133793 |
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Oct 2011 |
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TW |
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201320192 |
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May 2013 |
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TW |
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201428975 |
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Jul 2014 |
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TW |
|
Primary Examiner: Kim; Su C
Assistant Examiner: Wilbert; David S
Attorney, Agent or Firm: Hsu; Winston
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. application
Ser. No. 14/820,565 filed Aug. 7, 2015, the disclosure of which is
incorporated herein by reference in its entirety.
Claims
What is claimed is:
1. A method of fabricating a semiconductor device, comprising:
providing a semiconductor substrate having a first region and a
second region; forming a patterned mask in the first and second
regions of the semiconductor substrate; etching the semiconductor
substrate by using the patterned mask as an etch mask so as to form
a patterned structure on the surface of the semiconductor
substrate; forming a spacer disposed on the sidewall of the
patterned structure in the second region; etching the semiconductor
substrate by using the patterned mask and the spacer as an etch
mask so as to form a plurality of fin-shaped structures in the
first and second regions of the semiconductor substrate; forming a
first mask layer covering the fin-shaped structures in the first
region and a number of the fin-shaped structures in the second
region; forming an oxide layer on sidewalls of the fin-shaped
structures exposed from the first mask layer, the patterned mask
and the spacer in the second region; and removing the oxide layer,
so that the number of the fin-shaped structures in the second
region have base portions narrower than top portions and the other
fin-shaped structures in the second region have base portions wider
than top portions.
2. The method of claim 1, before the step of forming the spacer,
further comprising: depositing a material layer conformally
covering the fin-shaped structures; forming a second mask layer
covering the material layer in the second region so that the
material layer in the first region is exposed from the mask layer;
and etching the material layer in the first region by using the
second mask layer as an etch mask.
3. The method of claim 1, wherein the fin-shaped structures
comprise: a plurality of first fin-shaped structures disposed on a
semiconductor substrate, wherein each of the first fin-shaped
structures comprises: a base portion disposed on the semiconductor
substrate; and a top portion extending from the base portion of the
first fin-shaped structure, wherein the top portion of each of the
first fin-shaped structure has a first width; and a plurality of
second fin-shaped structures disposed on the semiconductor
substrate, wherein each of the second fin-shaped structures
comprises: the base portion disposed on the semiconductor
substrate; and the top portion extending from the base portion of
the second fin-shaped structure, wherein the top portion of each of
the second fin-shaped structure has the first width; and a recessed
region disposed on a sidewall of each of the second fin-shaped
structures.
4. The method of claim 1, wherein the fin-shaped structures are
separately disposed on the semiconductor substrate.
5. The method of claim 1, wherein the top portions of the first
fin-shaped structures have equal width.
6. The method of claim 1, further comprising: depositing a
dielectric layer to cover the fin-shaped structures; and performing
an etching process until each of the fin-shaped structures
partially protrudes from a top surface of the dielectric layer.
7. The method of claim 1, after the step of forming the fin-shaped
structures, further comprising: removing the spacer; depositing a
gate dielectric layer conformally covering the fin-shaped
structures; depositing a gate electrode layer covering the gate
dielectric layer; and patterning the gate dielectric layer and the
gate electrode layer to form a gate structure.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of
semiconductor devices, and more particularly to a fin-shaped
structure in a non-planar semiconductor device.
2. Description of the Prior Art
With the increasing miniaturization of semiconductor devices,
various multi-gate MOSFET devices have been developed. The
multi-gate MOSFETs are advantageous for the following reasons.
First, the manufacturing processes of the multi-gate MOSFET devices
can be integrated into traditional logic device processes easily,
and thus are more compatible. In addition, since the
three-dimensional structure of a multi-gate MOSFET increases the
overlapping area between the gate and the substrate, its channel
region can be controlled more effectively. This therefore reduces
drain-induced barrier lowering (DIBL) effect and short channel
effect (SCE). Moreover, the channel region is longer for a similar
gate length. Therefore, the current between the source and the
drain is increased. Besides, there is still a need to increase the
density of the semiconductor devices in an integrated circuit.
SUMMARY OF THE INVENTION
A semiconductor device is disclosed according to one embodiment of
the invention, the device which includes first fin-shaped
structures and second fin-shaped structures separately disposed on
a semiconductor substrate. Each of the first and second fin-shaped
structures includes a base portion and a top portion protruding
from the top portion. The base portions of the second fin-shaped
structures are wider than the top portions of the second fin-shaped
structures, and the top portions of the second fin-shaped
structures are as wide as the top portions of the first fin-shaped
structures. Each second fin-shaped structure further includes a
recessed region on its sidewall.
According to another embodiment of the present invention, a method
of fabricating a semiconductor device is also disclosed and
includes the following steps: providing a semiconductor substrate
having a first region and a second region, forming a patterned mask
in the first and second regions of the semiconductor substrate,
etching the semiconductor substrate by using the patterned mask as
an etch mask so as to form a patterned structure on the surface of
the semiconductor substrate, forming a spacer disposed on the
sidewall of the patterned structure in the second region, etching
the semiconductor substrate by using the patterned mask and the
spacer as an etch mask so as to form a plurality of fin-shaped
structures in the first and second regions of the semiconductor
substrate, forming a first mask layer covering the fin-shaped
structures in the first region, forming an oxide layer on sidewalls
of the fin-shaped structures exposed from the first mask layer, the
patterned mask and the spacer in the second region, and removing
the oxide layer, so that a number of the fin-shaped structures in
the second region have base portions narrower than top portions and
the other fin-shaped structures in the second region have the base
portions wider than the top portions.
These and other objectives of the present invention will no doubt
become obvious to those of ordinary skill in the art after reading
the following detailed description of the preferred embodiment that
is illustrated in the various figures and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 to FIG. 15 are schematic diagrams showing a method for
fabricating a semiconductor device according to preferred
embodiments of the present invention.
DETAILED DESCRIPTION
The invention will be described more fully hereinafter with
reference to the accompanying drawings, in which example
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the example embodiments set forth herein.
Rather, the disclosed embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope of the invention to those skilled in the art. In the
drawings, the size and relative sizes of layers and regions may be
exaggerated for clarity unless express so defined herein. Moreover,
each embodiment described and illustrated herein includes its
complementary conductivity type embodiment as well. Like numbers
refer to like elements throughout.
It will be understood that, although the terms first, second,
third, etc. may be used herein to describe various elements,
components, regions, layers and/or sections, these elements,
components, regions, layers and/or sections should not be limited
by these terms. These terms may be only used to distinguish one
element, component, region, layer and/or section from another
region, layer and/or section. Terms such as "first," "second," and
other numerical terms when used herein do not imply a sequence or
order unless clearly indicated by the context. Thus, a first
element, component, region, layer and/or section discussed below
could be termed a second element, component, region, layer and/or
section without departing from the teachings of the
embodiments.
Spatially relative terms, such as "inner," "outer," "beneath,"
"below," "lower," "above," "upper" and the like, may be used herein
for ease of description to describe one element and/or feature's
relationship to another element(s) and/or feature(s) as illustrated
in the figures. Spatially relative terms may be intended to
encompass different orientations of the device in use or operation
in addition to the orientation depicted in the figures. For
example, if the device in the figures is turned over, elements
described as "below" and/or "beneath" other elements or features
would then be oriented "above" the other elements or features.
Thus, the example term "below" can encompass both an orientation of
above and below. The device may be otherwise oriented (rotated 90
degrees or at other orientations) and the spatially relative
descriptors used herein interpreted accordingly.
The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting. As
used herein, the singular terms "a", "an," and "the" may be
intended to include the plural forms as well, unless the context
clearly indicates otherwise. The terms "comprises," "comprising,"
"includes" and/or "including" are inclusive and therefore specify
the presence of stated features, integers, steps, operations,
elements, and/or components, but do not preclude the presence or
addition of one or more other features, integers, steps,
operations, elements, components, and/or groups thereof. The method
steps, processes, and operations described herein are not to be
construed as necessarily requiring their performance in the
particular order discussed or illustrated, unless specifically
identified as an order of performance. It is also to be understood
that additional or alternative steps may be employed.
Example embodiments of the invention are described herein with
reference to cross-section illustrations that are schematic
illustrations of idealized embodiments (and intermediate
structures) of the invention. As such, variations from the shapes
of the illustrations as a result, for example, of manufacturing
techniques and/or tolerances, may be expected. Thus, the disclosed
example embodiments of the invention should not be construed as
limited to the particular shapes of regions illustrated herein
unless expressly so defined herein, but are to include deviations
in shapes that result, for example, from manufacturing. Thus, the
regions illustrated in the figures are schematic in nature and
their shapes are not intended to illustrate the actual shape of a
region of a device and are not intended to limit the scope of the
invention, unless expressly so defined herein.
Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and the present
disclosure, and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
Please refer to FIG. 1. At the beginning of the fabrication
process, a semiconductor substrate 100 having a first region RG 1
and a second region RG2 is provided. A mask layer, a sacrificial
pattern 130 and spacers 150 are disposed on the semiconductor
substrate 100. In detail, the mask layer may be single-layered or a
double-layered structure, such as a double-layered structure
including a first mask layer 110 and a second mask layer 120. The
sacrificial pattern 130 may have a width W2 and include a plurality
of stripe or ring-shaped features. The spacers 150 are disposed on
the sidewalls of the sacrificial pattern 130 and have widths
W1.
Because the sacrificial pattern 130 is fabricated by a
photolithographic process and an etching process, the minimum
dimension of the sacrificial pattern 130 are preferably greater
than or equal to "the minimum feature sizes that the current
exposure apparatus can achieve." Furthermore, because the spacers
150 are fabricated by depositing and etching a dielectric layer,
the dimensions of the spacers 150 may be less than "the minimum
feature sizes that the current exposure apparatus can achieve."
That is to say, the spacers 150 may have widths W1 less than the
widths W2 of the sacrificial pattern 130 and are therefore called
sub-lithographic features.
The substrate 100 may be a semiconductor substrate (such as a
silicon substrate), a silicon containing substrate (such as a
silicon carbide substrate), a III-V group-on-silicon (such as
GaN-on-silicon) substrate, a graphene-on-silicon substrate, a
silicon-on-insulator (SOI) substrate or an epitaxial layer
containing substrate. The first and second mask layers 110 and 120
are made of dielectric, such as silicon oxide or a silicon nitride,
but not limited thereto. The sacrificial pattern 130 may be made of
silicon material, III-V group semiconductors or other suitable
semiconductor materials, and preferably be made of polysilicon
material. The spacers 150 may be made of silicon oxide, silicon
nitride, oxynitride, silicon carbide or other suitable dielectric
materials different from the first and second mask layers 110 and
120 and the sacrificial pattern 130.
Please refer to FIG. 2. The sacrificial pattern 130 is removed
completely until the underlying second mask layer 120 is exposed.
Afterwards, by using the spacers as an etch mask, an etching
process is then carried out to sequentially transfer the pattern
consisting of the spacers 150 into the underlying second mask layer
120 and the first mask layer 110. The corresponding structure
fabricated by these processes is shown in FIG. 3.
Please refer to FIG. 3. A patterned first mask layer 111 and a
patterned second mask layer 121, also called patterned mask, may be
fabricated by the above-mentioned etching process, and portions of
the surface of the semiconductor substrate 100 are exposed from the
patterned mask. Because the pattern of the patterned mask is
fabricated by transferring the pattern of the spacers 150, the
widths of the patterned mask are preferably equal to or less than
the widths W1 of the spacers. Then, the semiconductor substrate 100
is further etched when covered by the patterned second mask layer
121 and the patterned first mask layer 111.
Please refer to FIG. 4. The patterned second mask layer 121 is
completely removed during the step shown in FIG. 3, and the
patterned first mask layer 11 is still left on the semiconductor
substrate 100. In this way, patterned structure, also called top
portions of the fin-shaped structure 120 and 122, is fabricated in
the first region RG 1 and the second region RG2 of the
semiconductor substrate 100, which has a predetermined first height
H1.
The processes of sequentially forming the sacrificial pattern,
forming the spacers, removing the sacrificial pattern and
transferring the pattern of the spacers to the underlying mask
layer may also be called "a spacer self-aligned double patterning
(SADP) process." Therefore, the patterned masks disclosed-above are
preferably "sub-lithographic features" and the dimensions of which
are less than "the minimum feature sizes that the current exposure
apparatus can achieve." In addition, other types of double
patterning processes may also be applied as an alternative of the
SADP process.
Please refer to FIG. 5. After the step shown in FIG. 4, a material
layer 132 is then conformally deposited on the surface of the
patterned first mask layer 111 and the surface of the top portions
of the fin-shaped structures 120 and 122. The composition of the
material layer 132 may be chosen from silicon nitride, silicon
oxide, silicon oxynitride, silicon carbide and so forth, and is
preferably different from that of the underlying semiconductor
substrate 100.
Please refer to the FIG. 6. A mask layer 134 is then formed to
cover the material layer 132, the patterned first mask layer 11 and
the top potions of the fin-shaped structures 122 in the second
region PG2. In this way, the material layer 132 in the first region
PG1 may be exposed from the mask layer 134. In detail, the
composition of the mask layer 134 is different that of the
underlying material layer 132 and is preferably made of
photoresist. Then, an etching process is carried out by using the
mask layer 134 as an etch mask. Through this process, the material
layer 132 in the first region RG1 can be removed completely.
Finally, the mask layer 134 is removed by a proper etching
process.
Please refer to FIG. 7. Another etching process 136 may be carried
out to form spacers 138 on the sidewalls of the patterned structure
in the second region RG2. In this embodiment, the widths of the
spacers 138 may be greater or less than, preferably less than,
those of the top portions 122 of the fin-shaped structures 122, but
are not limited thereto.
The processes of fabricating the spacers 138 are not limited to the
method disclosed above, that is, not limited to the steps of
forming the mask layer 134, removing the material layer 132 in the
first region RG1 when the semiconductor substrate 100 is covered by
the mask layer 134, removing the mask layer 134 and fabricating
spacers 138 in the second region RG2. The method may also be
replaced with other processes. For example, an etching process is
performed between the steps of depositing the material layer 132
and forming the mask layer 134 until the spacers 138 are fabricated
in the first region RG1 and the second region RG2. The mask layer
134 is then fabricated to cover the spacers 138 in the second
regions RG2. Another etching process is subsequently carried out
when the spacers 138 in the first region RG1 is covered by the mask
layer 134. In this way, the spacers 138 exposed form the mask layer
134 may be removed completely. Finally, the mask layer 134 is
removed and the structure shown in FIG. 7 is therefore
fabricated.
Please refer to FIG. 8. After the step shown in FIG. 7, another
etching process may be carried out by using the patterned first
mask layer 111 and the spacers 138 as an etch mask. Therefore, the
pattern consisting of the patterned first mask layer 111 and the
spacers 138 may be transferred to the semiconductor substrate 100
until base portions 140 and 142 of fin-shaped structure are
fabricated. Specifically, by applying the processes disclosed
above, a first fin-shaped structure 160 and a second fin-shaped
structure 162 may be respectively fabricated in the first region
RG1 and the second region RG2 of the semiconductor substrate 100.
The first fin-shaped structure 160 and the second fin-shaped
structure 162 are separately disposed on the semiconductor
substrate, and each of which, from its bottom to its top, includes
the base portion 140 and 142 and top portion 120 and 122 extending
from the base portion 140 and 142. Because the top portion 120 and
the base portion 140 of the first fin-shaped structure 160 are
fabricated by transferring the pattern of the patterned first mask
layer 111, and the top portion 122 of the second fin-shaped
structure 162 are fabricated by transferring the pattern of the
patterned first mask layer 111 and the spacers 138, the base
portion 142 of the second fin-shaped structure 162 is greater than
the width W1 of its top portion and the width W1 of the first
fin-shaped structure 160. Preferably, the base portion 142 and the
top portion 122 of the second fin-shaped structure 162 have smooth
sidewalls, and the width W1 of the top surface of the first
fin-shaped structure 160 is equal to the width W1 of the top
surface of the second fin-shaped structure 162. Besides, the
sidewall of the second fin-shaped structure 162 may further
includes a recess, also called a recessed region, adjacent to the
junction the base portion 142 and the top portion 122. The spacers
138 may be then completely removed after the base portions 140 and
142 of the fin-shaped structures are fabricated, and the top
surface of the base portion 142 is thereby exposed.
Please refer to FIG. 9. Following the step shown in FIG. 8, a step
of depositing a dielectric layer may be carried out until the first
and second fin-shaped structures 160 and 162 are covered by the
dielectric layer. Afterwards, a planarization and an etching back
process may be carried out sequentially to thereby fabricate a
shallow trench isolation structure 154. The top surface of the
shallow trench isolation structure 154 may have a predetermined
height so that the first and second fin-shaped structures 160 and
162 may protrude from the top surface of the shallow trench
isolation structure 154. In this embodiment, a top surface 155 of
the shallow trench isolation 154 may be higher or lower than,
preferably higher than, top surfaces 144 of the base portions 142,
but is not limited thereto. Afterwards, a gate dielectric layer 156
conformally covering the fin-shaped structures 160 and 162 and a
gate electrode layer 158 covering the gate dielectric layer 156 are
sequentially deposited. Finally, a gate structure may be fabricated
by patterning the gate dielectric layer 156 and the gate electrode
layer 158.
In the preceding description, the present disclosure is described
with reference to specifically exemplary embodiments thereof. It
is, however, evident that various modifications and changes may be
made thereto without departing from the broader spirit and scope of
the present disclosure, as set forth in the claims. In the
following paragraphs, a second embodiment and a third embodiment of
the present invention are disclosed.
FIG. 10 to FIG. 12 are schematic diagrams of a second embodiment of
the present invention. In this embodiment, the fin-shaped structure
in the second region is further oxidized, and the oxidized region
is removed. In this way, the base portion of the fin-shaped
structure in the second region can have a reduced width. The
detailed fabrication processes are disclosed below. Please refer to
FIG. 10. After the step shown in FIG. 8, a mask layer 170, such as
a photomask layer or a dielectric layer, is fabricated to cover the
first fin-shaped structure 160 in the first region RG1 and to
expose the base portion 142 of the second fin-shaped structure 162.
Subsequently, as shown in FIG. 11, an oxidation process is carried
out to by using the mask layer 170, the patterned mask 111 and the
spacers 138 as a mask. During the oxidation process, an oxide layer
172 may be formed on the surface of the semiconductor substrate 100
and on the sidewalls of the second fin-shaped structure 162 in the
second region RG2. In contrast, the first fin-shaped structure 160
is not oxidized during the processing because it is completely
covered by the mask layer 170. Afterwards, an etching process may
be carried out to remove the patterned mask 111, the spacers 138,
the mask layer 170 and oxide layer 172. The corresponding structure
is shown in FIG. 12, in which an etched area, also called a
recessed region, is formed on the sidewall of the lower portion of
the second fin-shaped structure 162 in the second region RG2. In
this way, the base portion 142 of the second fin-shaped structure
162 is narrower than the top portion 122 of the second fin-shaped
structure 162. The following process is similar to that shown in
FIG. 9.
According to the second embodiment disclosed in the above
paragraph, the mask layer 170 is formed after the formation of the
first fin-shaped structure 160, but is not limited thereto. For
example, the mask layer 170 may be formed between forming the
material layer 132 and forming the base portions 140 and 142 of the
fin-shaped structures.
FIG. 13 is a schematic diagram of a third embodiment of the present
invention. This embodiment incorporates the features of the first
and second embodiments in a way that the fin-shaped structures may
respectively have a recess or an etched area. Specifically, the
lower portions of the right-hand side second fin-shaped structure
162 in the second region RG2 may have etched areas 174, also called
recessed regions. Therefore, the base portion 142 of the second
fin-shaped structure 162 is narrower than the top portion 122 of
the second fin-shaped structure 162. In contrast, the sidewalls of
the left-hand side second fin-shaped structure 162 in the second
region RG2 may respectively have recesses 153, also called recessed
regions, adjacent to the junction of the base portion 142 and the
top portion 122. Therefore, the base portion 142 of the second
fin-shaped structure 162 is wider than the top portion 122 of the
second fin-shaped structure 162. This fin-shaped structure may be
formed by carrying out the oxidation process only to the right-hand
side second fin-shaped structure 162 in the second region RG2. As
shown in FIG. 15, unlike the one shown in FIG. 11, the mask layer
170 in the FIG. 15 is formed to cover the first fin-shaped
structure 160 in the first region RG1 and the left-hand side second
fin-shaped structure 162 in the second region RG2 to expose the
base portion 142 of the right-hand side second fin-shaped structure
162. Therefore, the base portion 142 of the right-hand side second
fin-shaped structure 162 may be oxidized with the mask layer 170,
the patterned mask 111 and the spacers 138 as a mask during the
oxidation process. In this way, the oxide layer 172 may be formed
on the surface of the semiconductor substrate 100 and only on the
sidewalls of the right-hand side second fin-shaped structure 162 in
the second region RG2. By carrying out an etching process to remove
the oxide layer 172 in FIG. 15, the fin-shaped structure of FIG. 13
may be obtained.
The process similar to that shown in FIG. 9 may be carried out so
as to fabricate the structure shown in FIG. 14. Please refer to
FIG. 14. The structure includes at least the shallow trench
isolation structure 154 and the gate structure. In detail, the top
surface of the shallow trench isolation structure 154 may have a
predetermined height so that the first and second fin-shaped
structures 160 and 162 may protrude from the top surface of the
shallow trench isolation structure 154. The gate structure also
includes the gate dielectric layer 156 conformally disposed on the
fin-shaped structures 160 and 162 and the gate electrode layer 158
disposed on the gate dielectric layer 156.
Those skilled in the art will readily observe that numerous
modifications and alterations of the device and method may be made
while retaining the teachings of the invention. Accordingly, the
above disclosure should be construed as limited only by the metes
and bounds of the appended claims.
* * * * *